1. Field of the Invention
The present invention relates generally to tensioners used with chain drives in automotive timing and power transmission applications. In particular, the present invention is related to a hydraulic chain tensioner system.
2. Description of Related Art
A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain, or similar power transmission devices, as the chain travels between a plurality of sprockets. In this device, the chain transmits power from a driving shaft to a driven shaft, so that part of the chain is slack and part of the chain is tight. Generally, it is important to impart and maintain a certain degree of tension in the chain to prevent noise, slippage, or the unmeshing of teeth in the case of a toothed chain. Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing, possibly causing damage or rendering the engine inoperative.
However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension. For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tensions. Reverse rotation of an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain and to ensure the necessary tension on the slack side of the chain.
Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. This lever arm must push toward the chain, tightening the chain when the chain is slack, and must be very rigid when the chain tightens.
To accomplish this result, a hydraulic tensioner typically comprises a rod or cylinder as a piston, which is biased in the direction of the chain by a tensioner spring. The piston is housed within a cylindrical housing, having an interior space which is open at the end facing the chain and closed at the other end. The interior space of the housing contains a pressure chamber in connection with a reservoir or exterior source of hydraulic fluid pressure. The pressure chamber is typically formed between the housing and the piston, and it expands or contracts when the piston moves within the housing.
Typically, valves are employed to regulate the flow of fluid into and out of the pressure chamber. For instance, an inlet check valve typically includes a ball-check valve that opens to permit fluid flow in to the pressure chamber when the pressure inside the chamber has decreased as a result of outward movement of the piston. When the pressure in the pressure chamber is high, the inlet check valve closes, preventing fluid from exiting the pressure chamber. The closing of the inlet check valve prevents the piston chamber from contracting, which in turn prevents the piston from retracting, achieving a so-called “no-return” function.
Many tensioners also employ a pressure relief mechanism that allows fluid to exit the pressure chamber when the pressure in the chamber is high, thus allowing the piston to retract in response to rapid increases in chain tension. In some tensioners, the pressure relief mechanism is a spring biased check valve. The check valve opens when the pressure exceeds a certain pressure point. Some tensioners may employ a valve which performs both the inlet check function as well as the pressure relief function.
Other mechanisms employ a restricted path through which fluid may exit the fluid chamber, such that the volume of flow exiting the fluid chamber is minimal unless the pressure in the fluid chamber is great. For instance, a restricted path may be provided through the clearance between the piston and bore, through a vent tube in the protruding end of the piston, or through a vent member between the fluid chamber and the fluid reservoir.
A hydraulic tensioner as used with a tensioner arm or shoe is shown in Simpson et al., U.S. Pat. No. 5,967,921, incorporated herein by reference. Hydraulic chain tensioners typically have a plunger slidably fitted into a chamber and biased outward by a spring to provide tension to the chain. A lever, arm or shoe is often used at the end of the plunger to assist in the tensioning of the chain. The hydraulic pressure from an external source, such as an oil pump or the like, flows into the chamber through passages formed in the housing. The plunger is moved outward against the arm by the combined efforts of the hydraulic pressure and the spring force.
When the plunger tends to move in a reverse direction (inward) away from the chain, typically a check valve is provided to restrict the flow of fluid from the chamber. In such a fashion, the tensioner achieves a so-called no-return function, i.e., movements of the plunger are easy in one direction (outward) but difficult in the reverse direction.
Blade and block tensioners have been used in the past to apply tension to chains. A block tensioner (1) as known in the prior art is shown in FIG. 1. The tensioner (1) has a piston (2) located within a housing (5). The springs (3) are located in a fluid chamber (4) within the piston (2).
An example of a blade tensioner is shown in FIG. 2. The conventional blade tensioner (10) includes a blade shoe (11) made of resin having a curved chain sliding face and numerous blade springs (21) preferably made of metallic material. The blade springs (21) are arranged in layers on the opposite side of the blade shoe (11) from the chain sliding face, and provide spring force to the blade shoe (11). The ends of each spring-shaped blade spring (21) are inserted in the indented portions (14) and (15) which are formed in the distal portion (12) and proximal portion (13) of the blade shoe (11), respectively.
A bracket (17) is provided for mounting the blade tensioner (10) in an engine. Holes (18) and (19) are formed in the bracket (17), and mounting bolts are inserted into these holes (18) and (19). A sliding face (16) contacts the distal portion of the blade shoe (11) and permits sliding. The slide face (16) is formed on the distal portion of the bracket (17). A pin (20) supports the proximal portion (13) of the blade shoe (11) so that it may move in either direction. The pin (20) is secured in the center of the bracket (17).
U.S. Pat. No. 5,647,811, shows a chain tensioner with an integrated tensioner and arm. The hydraulic tensioner in that patent is pressure fed.
Due to space restrictions, functionality or numerous other reasons, common tensioners such as the block type tensioners (FIG. 1) and blade type tensioners (FIG. 2) discussed above cannot be used in some applications. Therefore, there is a need in the art for an improved tensioner which can overcome the prior art shortcomings.